Diffusional quenching in isopentane (IP) glass at 77 K is demonstrated by the reduction of triphenylene phosphorescence lifetimes in the presence of 1,3-pentadiene and/or molecular oxygen. Fluorescence spectra and lifetimes of cis- and trans-1,2-di(1-methyl-2-naphthyl)ethene in IP glass at 77 K reveal that the cis -> trans photoisomerization leads to the trapping of unstable conformers of the trans isomer. The claim that IP at 77 K is not sufficiently viscous to trap unstable photoproduct conformers is invalidated.